The invention relates to a optical switch, and in particular to an optical switch and a method of controlling an optical switch for attenuation of an optical signal.
The control of optical power levels in optical communications systems is critical in obtaining optimum performance. The power level needs to be sufficient to establish a signal to noise ratio which will provide an acceptable bit error rate but without the power level exceeding a level at which limiting factors (e.g. the onset of non-linear effects) result in degradation of the signal or other co-propagating signals.
In wavelength division multiplexed (WDM) transmission, it is desirable to control the power of the individual optical channels or wavelengths. Channels could be controlled to provide constant system signal to noise ratio. One of the simplest methods of control is to maintain each of the power levels of the individual wavelength components (channels) at substantially the same level.
Alternate schemes provide different power levels for each channel, e.g. lower bit rate signals and/or signals propagating over a relatively short distance may be transmitted at lower powers.
The network components that the optical multiplex signal passes through typically have attenuation or amplification characteristics which vary with wavelength. In particular, optical amplifiers may not have a flat gain profile, providing different amounts of gain on different wavelengths. As an optical multiplex signal may pass through many amplifiers before the signal is regenerated, even small variations in gain flatness can produce large differences in the relative amplitudes of the individual optical channels.
It is also possible that wavelengths from different sources need to be placed in the same multiplex. An optical network may use optical time division multiplexing to compliment the wave division multiplexing functionality, routing different wavelength packets on different channels to different destinations. Consequently, optical signal packets or wavelengths from different sources may share common routes, and in order to ensure optimal transmission along those routes relatively frequent channel power equalisation may be necessary. If the paths traversed by the wavelengths through the network are different then even if the wavelengths started with the same amplitude at their respective origins, the amplitudes may be very different at the point they are placed on the same multiplex. If a channel is of much lower power than the adjacent channels interference from these channels due to fibre non-linearity will have a greater effect than if the channels had approximately equal powers.
It is therefore desirable to provide variable attenuators in communications system to allow the control of optical signal powers.
Variable optical attenuator systems are well-known in the telecommunications field. For instance, U.S. Pat. No. 5,956,437 describes an electrically controlled optical attenuator. The attenuator is formed by a mach-zehnder waveguide configuration with a variable refractive index element in one arm to modify the spectral characteristics of network to give a specific attenuation at a specific wavelength. Co-pending U.S. application Ser. No. 09/361,950 describes a method and apparatus for determining control signals of a filter for optimisation (i.e. attenuation) of the gain profile of an optical amplifier, and indicates how equalisation of an optical signal profile may be made over a relatively broad spectral range.
All of these instances require at least one attenuator/filter element to be incorporated into the system. If a range of channels need to be equalised (i.e. ensure that an approximately equal power is within each optical channel) then due to the roll off of the profiles of normal filters, it can be necessary to demultiplex a WDM system, equalise each channel individually using a separate attenuator, and then remultiplex the system. This significantly increases the component count and cost of any equalisation system.
The present invention aims to address such problems.
In one aspect, the present invention provides a method of controlling an optical switch comprising switching means arranged to switch an optical signal by redirection of the optical beam path of said signal, the method comprising controlled misalignment of the optical beam path so as to achieve a predetermined optical output power.
By controlling the misalignment of the optical beam path through the switch, the optical signal can be attenuated in a controlled manner. Utilising an optical switch in this format alleviates the requirement for separate optical attenuators to be incorporated into the system. If the optical system is being used as part of a WDM system, it is typical for the signal to be demultiplexed into the separate optical channels prior to input to the switch. If desired, each of the channels passing through the switch may be attenuated to whatever degree necessary to achieve the desired effect, e.g. equalisation of optical power across all channels. If the signal is demultiplexed into groups of channels, equalisation of power could also be applied to these groups of wavelengths.
If desired, normal operation of the switch could be performed with each of the signals attenuated by misalignment, thus permitting the optical power of any one or more signals to be increased by improving the beam alignment to the degree required to obtain the desired power.
Preferably, the method further comprises the steps of measuring the power of an optical signal; and controlling said switching means to misalign said optical beam path and achieve said predetermined output power.
Preferably, said step of measuring the optical signal power comprises at least one of measuring the input optical signal power and measuring the output optical signal power.
If the optical switch is calibrated such that a predetermined misalignment produces a predetermined attenuation, then only a single indication of the optical signal power is necessary. If desired, such a power measurement could be performed substantially upstream or downstream of the optical switch, at a different point within the network if the attenuation characteristics of any intervening components are known. Alternatively, both the input and the output optical signal to the switch could be measured in order to directly indicate the degree of the attenuation of the optical signal as it passes through the switch. This information could be used to provide a closed loop feedback control system to ensure that the desired degree of attenuation is achieved for each optical signal (or channel).
Preferably, the optical switch comprises at least two inputs and two outputs, and said measurement step comprises determining the relative ratios between the optical powers of at least any two optical signals.
Preferably, said optical switch comprises at least two inputs and two outputs arranged to switch the optical beam path of different wavelength optical signals, the method comprising misaligning respective optical beam paths so as to achieve a predetermined ratio of output optical power between at least any two of said different wavelength optical signals.
Preferably, said predetermined ratio is substantially unity. Hence, channel equalisation is achieved.
Preferably, said redirection of the optical beam path is achieved by reflection, refraction and/or diffraction.
In another aspect, the present invention provides a computer program on a machine-readable medium, said computer program being capable of performing a method of controlling an optical switch comprising switching means arranged to switch an optical signal by redirection of the optical beam path of said signal, the method comprising controlled misalignment of the optical beam path so as to achieve a predetermined optical output power.
In a further aspect, the present invention provides an optical switch comprising switching means arranged to switch an optical signal by redirection of the optical beam path of said signal, wherein said optical switch is arranged to misalign an optical beam path so as to provide a predetermined optical output power.
Preferably, the switch further comprises control means capable of receiving an input signal indicative of the power of an optical signal, the control means being arranged to control the functioning of said switching means for achieving misalignment of said optical beam path.
Preferably, the switch further comprises power measuring means arranged to provide a signal indicative of the power of an optical signal.
In another aspect, the present invention provides a telecommunications system comprising an optical switch comprising switching means arranged to switch an optical signal by redirection of the optical beam path of said signal, wherein said optical switch is arranged to misalign the optical beam path so as to provide a predetermined optical output power.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any aspects of the invention.